[CodeGen] Reuse stack space from unused function results

Summary:
Space on stack allocated for unused structures returned by functions was unused
even when it's lifetime didn't intersect with lifetime of any other objects that
could use the same space.

The test added also checks for named and auto objects.  It seems to make sense
to have this all in one place.

Reviewers: aadg, rsmith, rjmccall, rnk

Reviewed By: rnk

Subscribers: asl, cfe-commits

Differential Revision: http://reviews.llvm.org/D9743

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237385 91177308-0d34-0410-b5e6-96231b3b80d8